人体热释电红外传感器PIR原理

人体热释电红外传感器PIR原理详解

在电子防盗、人体探测器领域中，被动式热释电红外探测器的应用非常广泛，因其价格低廉、技术性能稳定而受到广大用户和专业人士的欢迎。

被动式热释电红外探头的工作原理及特性：

人体都有恒定的体温，一般在37度，所以会发出特定波长10μm左右的红外线，被动式红外探头就是靠探测人体发射的10μm左右的红外线而进行工作的。人体发射的10μm 左右的红外线通过菲涅尔滤光片增强后聚集到红外感应源上。红外感应源通常采用热释电元件，这种元件在接收到人体红外辐射温度发生变化时就会失去电荷平衡，向外释放电荷，后续电路经检测处理后就能产生报警信号。

(1)这种探头是以探测人体辐射为目标的。所以热释电元件对波长为10μm 左右的红外辐射必须非常敏感。

(2)为了仅仅对红外辐射敏感，在它的辐射照面通常覆盖有特殊的菲涅尔滤光片，使环境的干扰受到明显的控制作用。

(3)被动红外探头，其传感器包含两个互相串联或并联的热释电元。而且制成的两个电极化方向正好相反，环境背景辐射对两个热释元件几乎具有相同的作用，使其产生释电效应相互抵消，于是探测器无信号输出。

(4)一旦人侵入探测区域内，人体红外辐射通过部分镜面聚焦，并被热释电元接收，但是两片热释电元接收到的热量不同，热释电也不同，不能抵消，经信号处理而报警。

(5)菲涅尔滤光片根据性能要求不同，具有不同的焦距(感应距离)，从而产生不同的监控视场，视场越多，控制越严密。

被动式热释电红外探头的优缺点：

优点：

本身不发任何类型的辐射，器件功耗很小，隐蔽性好。价格低廉。

缺点：

◆容易受各种热源、光源干扰

◆被动红外穿透力差，人体的红外辐射容易被遮挡，不易被探头接收。

◆易受射频辐射的干扰。

◆环境温度和人体温度接近时，探测和灵敏度明显下降，有时造成短时失灵。

抗干扰性能：

1.防小动物干扰

探测器安装在推荐地使用高度，对探测范围内地面上地小动物，一般不产生报警。

2.抗电磁干扰

探测器的抗电磁波干扰性能符合GB10408中4.6.1要求，一般手机电磁干扰不会引起误报。

3.抗灯光干扰

探测器在正常灵敏度的范围内，受3米外H4卤素灯透过玻璃照射，不产生报警。

红外线热释电传感器的安装要求：

红外线热释电人体传感器只能安装在室内，其误报率与安装的位置和方式有极大的关系，正确的安装应满足下列条件：

1.红外线热释电传感器应离地面

2.0-2.2米。

2.红外线热释电传感器远离空调, 冰箱，火炉等空气温度变化敏感的地方。

3.红外线热释电传感器探测范围内不得隔屏、家具、大型盆景或其他隔离物。

4.红外线热释电传感器不要直对窗口，否则窗外的热气流扰动和人员走动会引起误报，有条件的最好把窗帘拉上。红外线热释电传感器也不要安装在有强气流活动的地方。

红外线热释电传感器对人体的敏感程度还和人的运动方向关系很大。热释电红外传感器对于径向移动反应最不敏感, 而对于横切方向(即与半径垂直的方向)移动则最为敏感. 在现场选择合适的安装位置是避免红外探头误报、求得最佳检测灵敏度极为重要的一环。

菲涅尔透镜

概述

菲涅尔透镜多是由聚烯烃材料注压而成的薄片，镜片表面一面为光面，另一面刻录了由小到大的同心圆。菲涅尔透镜的在很多时候相当于红外线及可见光的凸透镜，效果较好，但成本比普通的凸透镜低很多。菲涅尔透镜可按照光学设计或结构进行分类。菲涅尔透镜作用有两个：一是聚焦作用；二是将探测区域内分为若干个明区和暗区，使进入探测区域的移动物体能以温度变化的形式在PIR（被动红外线探测器）上产生变化热释红外信号。

作用

菲涅尔透镜利用透镜的特殊光学原理，在探测器前方产生一个交替变化的“盲区”和“高灵敏区”，以提高它的探测接收灵敏度。当有人从透镜前走过时，人体发出的红外线就不断地交替从“盲区”进入“高灵敏区”，这样就使接收到的红外信号以忽强忽弱的脉冲形式输入，从而强其能量幅度。

菲涅尔透镜

菲涅尔透镜，简单的说就是在透镜的一侧有等距的齿纹，通过这些齿纹，可以达到对指定光谱范围的光带通(反射或者折射)的作用。传统的打磨光学器材的带通光学滤镜造价昂贵。菲涅尔透镜可以极大的降低成本。

典型的例子就是PIR。PIR广泛的用在警报器上。如果你拿一个看看，你会发现在每个PIR 上都有个塑料的小帽子。这就是菲涅尔透镜。小帽子的内部都刻上了齿纹。这种菲涅尔透镜可以将入射光的频率峰值限制到10微米左右（人体红外线辐射的峰值）。

菲涅耳透镜可以把透过窄带干涉滤光镜的光聚焦在硅光电二级探测器的光敏面上，菲涅尔透镜不能用任何有机溶液(如酒精等)擦拭，除尘时可先用蒸馏水或普通净水冲洗，再用脱脂棉擦拭。

How Infrared motion detector components work

Infrared Radiation_____________

Infrared radiation exists in the electromagnetic spectrum at a wavelength that is longer than visible light. It cannot be seen but it can be detected. Objects that generate heat also generate infrared radiation and those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4um. Infrared in this range will not pass through many types of material that pass visible light such as ordinary window glass and plastic. However it will pass through, with some attenuation, material that is opaque to visible light such as germanium and silicon. An unprocessed

silicon wafer makes a good IR window in a weatherproof enclosure for outdoor use. It also provides additional filtering for light in the visible range. 9.4um infrared will also pass through polyethylene which is usually used to make Fresnel lenses to focus the infarared onto sensor elements.

Pyroelectric Sensors_____________

The pyroelectric sensor is made of a crystalline material that generates a surface electric charge when exposed to heat in the form of infrared radiation. When the amount of radiation striking the crystal changes, the amount of charge also changes and can then be measured with a sensitive FET device built into the sensor. The sensor elements are sensitive to radiation over a wide range so a filter window is added to the TO5 package to limit detectable radiation to the 8 to 14mm range which is most sensitive to human body radiation.

Typically, the FET source terminal pin 2 connects through a pulldown resistor of about 100 K to ground and feeds into a two stage amplifier having signal conditioning circuits. The amplifier is typically bandwidth limited to below 10Hz to reject high frequency noise and is followed by a window comparator that responds to both the positive and negative transitions of the sensor output signal. A well filtered power source of from 3 to 15 volts should be connected to the FET drain terminal pin 1.

The PIR325 sensor has two sensing elements connected in a voltage bucking configuration. This arrangement cancels signals caused by vibration, temperature changes and sunlight. A body passing in front of the sensor will activate first one and then the other element whereas other sources will affect both elements simultaneously and be cancelled. The radiation source must pass across the sensor in a horizontal direction when sensor pins 1 and 2 are on a horizontal plane so that the elements are sequentially exposed to the IR source. A focusing device is usually used in front of the sensor

The figure below shows the PIR325 electrical specifications and layout in its TO5 package. Note the wide viewing angle without an external lens.

This is a typical application circuit that drives a relay. R10 and C6 adjust the amount of time that RY1 remains energized after motion is detected. Download PDF drawing.

Fresnel Lens_____________

A Fresnel lens (pronounced Frennel) is a Plano Convex lens that has been collapsed on itself to form a flat lens that retains its optical characteristics but is much smaller in thickness and therefore has less absorption losses.

Our FL65 Fresnel lens is made of an infrared transmitting material that has an IR transmission range of 8 to 14um which is most sensitive to human body radiation. It is designed to have its grooves facing the IR sensing element so that a smooth surface is presented to the subject side of the lens which is usually the outside of an enclosure that houses the sensor.

The lens element is round with a diameter of 1 inch and has a flange that is 1.5 inches square. This flange is used for mounting the lens in a suitable frame or enclosure. Mounting can best and most easily be done with strips of Scotch tape. Silicone rubber can also be used if it overlaps the edges to form a captive mount.

The FL65 has a focal length of 0.65 inches from the lens to the sensing element. It has been determined by experiment to have a field of view of approximately 10 degrees when used with a PIR325 Pyroelectric sensor.

This relatively inexpensive and easy to use Pyroelectric Sensor and Fresnel Lens can be used in a variety of science projects, robots and other useful devices.

Focusing devices for pyroelectric infrared sensors

Pyroelectric infrared sensors

First we will look at a pyroelectric infrared sensor and see how it is made and why a focusing device is necessary. A commonly used pyroelectric infrared sensor has two sensing elements internally connected in a voltage bucking configuration.